In satellite broadcasting, radio waves in the 12-GHz band width are transmitted from geostationary broadcasting satellites located approximately 36,000 km above the equator, and the radio waves are received by high gain parabola antennas on the ground. The waves collected by the parabola antenna are frequency converted to the 1-GHz band by a low noise convertor, and then carried indoors by a coaxial cable. This frequency conversion is conventionally referred to as primary conversion. The 1-GHz band television signal conducted indoors is again frequency converted to a 400-MHz second intermediate frequency signal. This secondary frequency signal is signal processed for FM demodulation to obtain the video and audio signals. The secondary frequency convertor effects a tuning function, selecting one wave from the 1-GHz primary frequency signal to obtain the secondary frequency signal.
Devices conventionally used for secondary frequency signal convertors include transistor mixers, which use transistors, and diode mixers, which use diodes and balun-type transformers. While the transistor mixer offers the advantage of conversion gain, diode mixers are more commonly used due to their low intercept point.
A conventional device is described below with reference to the accompanying diagrams, FIG. 1 is an equivalent circuit diagram for of a frequently used double balun transformer mixer. In FIG. 1, reference numerals 4 and 5 denote balun transformers, and reference numerals 6, 7, 8 and 9 denote diodes. There is a degree of inherent isolation in this mixer because the ports are balanced, and the design of the isolation filter is simplified because the leakage to the intermediate frequency output terminal 2 of the local oscillation signal applied to terminal 3 and the high frequency input signal applied to terminal 1 is low.
FIG. 2 is an equivalent circuit diagram for a single balun transformer mixer. In FIG. 2, reference numeral 5 denotes a balun transformer, and reference numerals 6 and 7 denote diodes. The features of the single balun transformer mixer include a simpler construction and lower cost than a double balun transformer mixer, and low conversion loss because of the smaller number of diodes and balun transformers. On the other hand, because the high frequency input terminal 1 and the intermediate frequency output terminal 1 are not isolated, a filter with better attenuation characteristics is required (Ref.: "Koushuha Kairo no Sekkei to Jissai (Design and practice of high frequency circuits)", by Yukihiko Miyamoto, issued October 1987).
A balun transformer is generally comprised of a compact toroidal core and a so-called "spectacle" core, constructed by wrapping a 0.1-0.3 mm diameter wire around the core 5' as shown in FIG. 3 (Ref.: "Minseiyou Koushuha Device Handbook (Handbook of high frequency devices for consumer applications)", NEC Co., Ltd, June 1986). In FIG. 3, reference numerals 30 and 31 denote unbalanced terminals, reference numerals 32 and 33 denote balanced terminals, and reference numeral 34 denotes a ground terminal.
An example of a mixer comprised of the balun transformer and diodes as thus described mounted on a printed circuit board is shown in FIGS. 4a and b. The frequency characteristic of the conversion loss, which is the most important characteristic of a mixer circuit, is approximately 6-7 dB as shown in FIG. 5, which is slightly lower than the 8-10 dB conversion loss in a double balanced mixer.
The operation of a single balun mixer constructed as described above is discussed hereinbelow. The function of the balun transformer 5 shown in FIG. 2 is to unbalance-balance convert the local oscillator output signal applied to the local oscillator input terminal 3, and insert the converted signal to diodes 6 and 7. What is important here is that the balun transformer 5 is balanced, which means that the phase of the local oscillator output component at points A and C is each .+-.90.degree. to point B (a center tap of the coil). If the transformer is thus balanced, the separated local oscillator output signal is canceled respectively at the high frequency input terminal and the intermediate frequency output terminal 1, i.e., isolation is good. A Schottky barrier diode is normally used for the diodes 6 and 7 because a sufficiently high speed switching characteristic is required for high frequency signals. As these diodes are switched on/off by the local oscillator signal, an equivalent circuit as shown in FIG. 6 can be constructed. (Ref.: ("Toroiguru koa Katsuyou Hyakka (Encyclopedia of toroidal core applications)", p, 270, Hideo Yamamura, issued January 1983).
Referring to FIG. 6, because the high frequency input signal applied to terminal 1 is switched by the local oscillator signal, an output having the components as shown in the following formula appears at the intermediate frequency output terminal 1. EQU f.sub.IFm,n =.+-.mf.sub.RF .+-.nf.sub.LO
where
m, n=0, 1, 2, 3 . . .
f.sub.IF : intermediate frequency
f.sub.RF : high frequency input frequency
f.sub.LO : local oscillator signal frequency.
In the above equation, m=n=0 is the DC component which is normally blocked. In addition, a high output is removed by the LPF connected to the intermediate frequency output terminal, and the difference component of the frequency of the local oscillator signal and the high frequency input signal is extracted as the intermediate frequency signal, thereby effecting a frequency convertor function.
However, in a conventional mixer as shown in FIG. 4, it is necessary to electrically connect by means of soldering the wires which act as the input/output terminals of the balun transformer 5 after mounting the balun transformer 5 to the printed circuit board 14 by inserting the wires into holes provided in the printed circuit board 14. The wires used for the balun transformer 5 are light copper wires of a 0.1-0.3 mm diameter and coated with a polyurethane or other protective coating. These soft wires are hard to insert in the circuit board holes during mounting and can be easily bent, thereby resulting in variations in conversion gain and other performance characteristics.